1. Field of the Invention
The present invention relates to anhydrous mirtazapine crystals and a process for preparing the same, and crystals of a mirtazapine hydrate and a process for preparing the same. More specifically, the present invention relates to anhydrous mirtazapine crystals having low hygroscopic properties, which are useful as an antidepressant, and a process for preparing the same, and crystals of a mirtazapine hydrate, which are useful as a preparation intermediate for the anhydrous mirtazapine crystals, and a process for preparing the same.
2. Discussion of the Related Art
As a process for increasing the purity of mirtazapine, there has been proposed a process for recrystallizing mirtazapine from a petroleum ether or the like (U.S. Pat. No. 4,062,848).
However, there are some defects in this process that impurities are precipitated in an oily state when a crude mirtazapine having a purity of 95 to 99% or so is used, so that the crystallization of mirtazapine is inhibited, and that it would be difficult to crystallize mirtazapine having a high purity.
In addition, since the crystals of mirtazapine have hygroscopic properties, there are some defects in the crystals that they cannot be handled and stored if they are not under dry conditions.
Accordingly, there have been earnestly desired the development of a process capable of efficiently preparing mirtazapine having a high purity from a crude mirtazapine, and the development of mirtazapine crystals having low hygroscopic properties.
The present invention has been accomplished in view of the prior art described above. An object of the present invention is to provide a process capable of efficiently preparing a high-purity mirtazapine from a crude mirtazapine, and anhydrous mirtazapine crystals having low hygroscopic properties and a process for preparing the same; and crystals of a mirtazapine hydrate, which are useful as a preparation intermediate for the anhydrous mirtazapine crystals and a process for preparing the same. These and other objects of the present invention will be apparent from the following description.
According to the present invention, there are provided:
(1) low-hygroscopic anhydrous mirtazapine crystals having a hygroscopic degree of not more than 0.6% by weight when the crystals are stored in the air having a relative humidity of 75% at 25xc2x0 C. under atmospheric pressure for 500 hours;
(2) a process for preparing low-hygroscopic anhydrous mirtazapine crystals having a hygroscopic degree of not more than 0.6% by weight when the crystals are stored in the air having a relative humidity of 75% at 25xc2x0 C. under atmospheric pressure for 500 hours, comprising drying crystals of mirtazapine hydrate; and
(3) a crystal of a mirtazapine hydrate represented by the formula (I): 
wherein n is an integer of 1 to 5, and a process for preparing the same.